Variable optical attenuators (VOAs) are common components used in optical communication networks. For example, a power level of an optical signal may need to be reduced to meet certain operational requirements. In particular, it may be desirable to maintain a uniform power level for different WDM channels in a communication system. In an optical amplifier, it may be desired to maintain a uniform gain profile over a wavelength range of interest. In some cases, it may be necessary to control (limit) the power level of an output signal to remain within a predetermined dynamic range of an optical detector. In any of these situations (or many others), there is a need to provide “active” adjustment of the output power level of one optical signal relative to others. Variable optical attenuators serve this purpose.
One common VOA technology is based on the use of a micro-electro-mechanical (MEMS) device to perform beam steering in a manner that controls the power coupled into an output signal path (i.e., controls the attenuation between the input and output paths). The typical VOA component includes an input optical fiber, lens, MEMS tilting mirror and an output optical fiber. The lens focuses the input light onto the MEMS tilting mirror, with the light reflected from the mirror then directed into the output fiber. A voltage is applied to the MEMS tilting mirror, where the voltage amplitude controls the tilt angle of the mirror. By varying the voltage (and, therefore, the tilt angle), the position of the reflected spot on the output fiber is varied. With the output spot aligned to the center of the output fiber's core, the attenuation is minimal (limited only by insertion loss, typically about 0.5 dB). As the output spot of the beam of reflected light is misaligned relative to the output fiber core (that is, as the tilt angle of the MEMS mirror changes), the amount of light launched into the output fiber core is reduced, providing a higher level of attenuation. The maximum attenuation can be 30 dB or higher, mainly limited by the tilt range of the mirror.
Control of the attenuation provided by a VOA is generally achieved by using an external system of discrete components for monitoring both the power going into the VOA and the power exiting the VOA. This external monitoring system typically includes input and output power monitors, with each separate power monitor including an external, fiber-based tap coupler and an associated photodiode. A determination of the relative difference in measured input and output powers is fed back to a controller that sets the voltage applied to the MEMS mirror, and thus the tilt angle required to achieve the required loss (attenuation) of the VOA.
With an on-going emphasis to provide “small form factor” optical components, it is desirable to integrate the monitoring function with the VOA itself. However, combining the individual, discrete components of a conventional monitoring system into the constrained size of a packaged VOA is problematic. Thus, a need remains in the art for a VOA configuration that is efficient and accurate, yet is able to meet the packaging limitations of small form factor configurations.